The present invention relates to a color image processing apparatus and, more particularly, to a color image-processing apparatus for use in a color copier.
Japanese laid-open Patent Publication No. 60-38796 discloses a color copier comprising: a color line-image sensor for optically reading a color image of an original document to be copied, and generating color image signals; a matrix circuit for converting color image signals into brightness and color-difference signals; and a color conversion circuit for converting the brightness and color-difference signals into ink-amount control signals, to control the amounts of color inks (e.g., yellow, red, magenta, and cyan) used in a color printer. The color conversion circuit is comprised of a data table formed of a read-only memory (ROM).
With the apparatus as described above, in order to faithfully reproduce dark colors which are visually very important, the color conversion circuit (ROM table) must have a considerably large memory capacity. Conversely, to reproduce light colors, the contents of the ROM table are quantized much more finely than required. This means that substantially the same information is uselessly stored in the ROM table.
With the prior art, it is possible to change hues by changing matrix conversion coefficients stored in the matrix circuit. To greatly change a hue, however, if the matrix conversion coefficients are simply changed, then the change in hue would become different for each color, and the saturation of the color would also be degraded. As a result, a color image would be reproduced whose hues would look unnatural.
The color image sensor is comprised of a CCD (charge-coupled device). In practice, the image sensor comprises a plurality of CCD chips to cover the size (width) of the document for copying. The CCD chips are arranged along the widthwise direction (the main scanning direction) of the document. With such a configuration, however, variations in characteristics among chips (especially variations in spectral characteristics of color filter arrays provided for the CCD chips) are inevitable, thus resulting in variations in the output signals of the CCD chips. As a result, noticeable differences between reproduced colors would appear at those portions of a reproduced image corresponding to boundaries between the CCD chips.
A copending U.S. patent application Ser. No. 753,260 filed Sept. 9, 1985, now U.S. Pat. No. 4,661,843 and assigned to the same assignee as this application discloses a color image copying apparatus which comprises a nonlinear conversion circuit between a matrix circuit and a color conversion circuit for the purpose of changing, for example, the density of a reproduced color image.